Process for after-treating metal surfaces coated with crystalline anticorrosion layers



United States Patent No Drawing. Filed Jan. 23, 1962, Ser. No. 168,261 Claims priority, application Germany, Feb. 8, 1961, F 33 161 3 Claims. (Cl. 148--6.15)

The present invention provides a process and a composition for the after-treatment of metal surfaces coated with crystalline anticorrosion layers in order to improve the protection against corrosion. The invention is particularly directed to the after-treatment of said metal surfaces by treating them with a solution containing aliphatic phosphonic acids having more than 5 carbon atoms.

It is known to protect metal surfaces against corrosion by applying thereto an inorganic covering layer, for example a phosphate layer. Due to their porosity crystalline layers of this kind alone do not ensure an effective protection against corrosion. It is known that iron and steel surfaces are well protected against corrosion only when the free area of the pores of the coating covers less than 0.01% of the total surface. Such a dense protection layer is, for example, the natural oxide film on aluminiurn and chromium. As compared therewith, phosphate layers have a pore area which is about 50 times larger, namely about 0.5%. Consequently, it is necessary to subject protecting layers of this kind to an after-treatment.

For this reason the anticorrosion layers on metal surfaces are Often oiled. This is only possible, however, when the metal parts are not to be lacquered. In general, the layers are passivated with dilute solutions of hexavalent chromium compounds. The use of such compounds is, however, disadvantageous in that the waste water must be freed from the toxic chromium compounds.

It has now been found that the above disadvantages can be avoided and metals, especially such as zinc, aluminium, iron and steel can be well protected against corrosion by inorganic crystalline covering layers, preferably phosphate layers, when the metal parts provided with the crystalline layers are after-treated with solutions containing aliphatic phosphonic acids having more than 5 carbon atoms, and subsequently dried.

The solutions for the after-treatment suitably contain the aliphatic phosphonic acids in an amount in the range of to 10 mol per liter and preferably 10* to 10 mol per liter.

The short-chain aliphatic phosphonic acids are suitably applied in the form of aqueous solutions. Since, however, the solubility of these compounds in water decreases as the chain length increases and products having a long chain are practically insoluble in water it is of advantage to dissolve said substances in mixtures of water with organic solvents, especially aliphatic ketones or aliphatic alcohols having 1-5 carbon atoms. Especially suitable organic solvents are acetone and isopropanol.

It is likewise possible to use solutions of phosphonic acid in pure organic solvents, but only in case the metal parts are still moist from the application of the protecting layer, for example the phosphate layer. The metal parts treated with the solutions of the invention can be dried much more rapidly.

The present invention furthermore provides a composition for the preparation of the after-treatment solutions which composition consists of a concentrate of the ali- "ice phatic phosphonic acids having more than 5 carbon atoms, water and/ or organic solvents, for example alcohols having 1-5 carbon atoms, or ketones.

By using the composition for preparing the after-treatment solutions in the form of a concentrate, it is possible from the beginning to maintain a suitable mixing ratio so that when introducing the concentrate into water a bath is obtained which contains a sufiicient amount of compounds required for a successful treatment.

The known after-treatment processes which use compounds of chromic acid necessitate a waste water purification on account of the toxicity of the hexavalent chromium compounds, whereas the waste waters originating from the treatment baths of the invention are physiologically harmless to fish.

More particularly, the process of the present invention is carried out in a manner such that the degreased metal parts, which have subsequently been provided with an anticorrosion layer, are after-treated for a short period of time, for example, a few seconds up to some minutes, with the described solutions in usual manner, for example by spraying, immersing and flooding, and then dried.

An upper limitation of the treatment time is only important when such crystalline layers are after-treated and are attacked to a substantial degree, i.e. dissolved, by the aliphatic, long-chain phosphonic acids used according to the invention. The low concentrations used in the process of the invention dissolve the crystalline coating layers in most cases to a very small degree only and at a treatment time of less than about 15 minutes by the immersion or spraying process a dissolution of the crystalline layer cannot be detected. The lower limit of the treatment time is practically given by a complete wetting of the after-treated surface with the solutions according to the invention.

The temperature of the after-treatment solution used according to the invention is not critical and only limited by the boiling and freezing point of said solution.

The following examples serve to illustrate the invention, but they are not intended to limit it thereto.

Example 1 Steel plates for motor car bodies were degreased at boiling temperature with a commercial, strongly alkaline cleansing agent rinsed with water and treated for ten minutes at a bath temperature of 60 C. in a phosphatization bath containing an oxidizing accelerator and having 35 points (the total acidity, as determined by standard titration procedures, wherein a 10 ml. sample of the solution is titra-ted with 0.1 normal sodium hydroxide to a phenolphthalein end point, is represented by the number of milliliters of base used, this number being referred to as points). The steel plates were thus provided with a firmly adhering layer of zinc phosphate.

One part of the phosphatized plates was immersed for 15 seconds in a 2.7 10- molar solution of eicosanephosphonic acid (C H PO I-I in a water/isopropanol mixture (proportion by weight 1:1) and then dried in warm air. The other part of the phosphatized plates was treated for 45 seconds with a commercial after-passivation solution containing about 0.03% of chromic acid in addition to small amounts of phosphoric acid.

The plates treated with the solution of the invention as well as the plates treated with chromic acid were then provided with an alkyd resin baking lacquer and the plates were subjected to the usual standardized corrosion and lacquer adherence tests.

The plates treated by the process of the invention showed an excellent protection against corrosion which was superior to that produced on the comparative plates with chromic acid. The values obtained for the lacquer adherence were about equal for either type of plates.

The same advantageous results were obtained when instead of the defined eicosane-phosphonic acid solution the following solutions were used:

(a) 4.5 x 10- mol/liter decane-phosphonic acid in a water/isopropanol mixture (proportion by volume 2:1

(b) 2.6)(10 mol/liter octane-phosphonic acid in a water/ acetone mixture (proportion by volume 3:1) (c) 6.3 10 mol/ liter hexane-phosphonic acid in water (d) mol/ liter hexane-phosphonic acid in a water/ isopropanol mixture (proportion by volume 1:1)

(e) 10 mol/ liter octane-phosphonic acid in a met-hanol/ water mixture (proportion by volume 1:1)

(f) 5 10 mol/ liter hexane-phosphonic acid in a butanol/water mixture (proportion by weight 10:1).

Example 2 Steel plates for motor car bodies were degreased and phosphatized as described in Example 1. The plates leaving the phosphatization bath were rinsed with water and immersed in a solution of 3X10 mol per liter of hexane-phosphonic acid in isopropanol. The drying of the plates with hot air took a much shorter time than the drying of the plates treated with aqueous solutions according to Example 1. The plates treated in this manner showed the same protection against corrosion and the same good lacquer adherence as the plates of Example 1.

We claim:

1. A process for the after-treatment of phosphatizcd ferrous surfaces in order to improve the corrosion protection, which comprises treating said coated metal surfaces with an aqueous solution containing about 10 mol per liter solution to about 10- mol per liter solution of a saturated aliphatic phosphonic acid containing more than 5 carbon atoms, and subsequently drying said metal surfaces.

2. A process as claimed in claim 1, wherein said solution contains as an additional solvent 21 member from the group consisting of water aliphatic alcohols containing 1 to 5 carbon atoms, and aliphatic ketones containing less than 5 carbon atoms.

3. A process for the after-treatment of phosphatized surfaces of a metal selected from the group consisting of zinc, aluminum, iron and steel in order to improve the corrosion protection, which comprises treating said coated metal surfaces with an aqueous solution containing about 10- mol per liter solution to about 10- mol per liter solution of a saturated aliphatic phosphonic acid containing more than 5 carbon atoms, and subsequently drying said metal surface.

Kosolapoff: I. Am. Chem. Soc., vol. 67 (July 1945), pp. 1 -1 182.

Karrer: Organic Chemistry, Elsevier Amsterdam, page 126.

ALFRED L. LEAVITT, Primary Examiner.

RICHARD D. NEVIUS, MURRAY KATZ, Examiners.

R. S. KENDALL, Assistant Examiner. 

1. A PROCESS FOR THE AFTER-TREATMENT OF PHOSPHATIZED FERROUS SURFACES IN ORDER TO IMPROVE THE CORROSION PROTECTION, WHICH COMPRISES TREATING SAID COATED METAL SURFACES WITH AN AQUEOUS SOLUTION CONTAINING ABOUT 10-5 MOL PER LITER SOLUTION TO ABOUT 10-1 MOL PWER LITER SOLUTION OF A SATURATED ALIPHATIC PHOSPHONIC ACID CONTAINING MORE THAN 5 CARBON ATOMS, AND SUBSEQUENTLY DRYING SAID METAL SURFACES. 